The common procedure for making a lamp with a backup filament is to have both filaments enclosed in the same envelope. An external mechanism switches from one filament to the other when the first filament fails. The two filaments are generally positioned in the lamp envelope to not contact one another during lamp operation, since filaments sag or become slinky with heat, and age. Filaments swing, and vibrate with lamp motions, so the filaments are also positioned to not contact one another during lamp motions. Positioning and motion control have usually meant internal metal supports to restrict filament motion, but internal supports require additional manufacturing expense, and internal attachments to or through the envelope. Internal suppors may weaken the envelope or seal. There is then a need to design a double filament lamp without internal metal filament supports.
In double filamented lamps, the first and second filaments, and the filament supports are frequently close. The light from the lit filament falls on the unlit filament, any filament remains from an exhausted filament, and any filament supports, casting shadows on the lighted subject. The filament, and support locations are therefore normally designed to achieve the least shadow for a particular use; however, the easiest solution of placing the filaments side by side in parallel invites vibrational contact, and the next easiest solution of separating the filaments leads to large envelopes, extended support structures, and generally more complex and expensive internal structures. There is then a need to provide a simple inexpensive means of locating and supporting two filaments that reduces shadowing, and complex support.
Double filamented lamps enclosed in a single envelope can also fail completely when the first filament fails normally due to age, but releases filament fragments that may fall on the back up filament. The second or back up filament then either lights along with the fallen material in an odd display, or fails due to shorting or mechanical stress. There is then a need for a double filament lamp where a failed filament cannot interfere with the operation of the back up filament.
In an incandescent lamp, and even in tungsten halogen lamps where the halogen cycle is functioning, there is some deposition of the filament material on the envelope walls as the filament ages with use. Tungsten deposition occurs most readily where the inside of the lamp envelope is coolest. In a double filamented lamp, where one filament is positioned away from a second filament for mechanical or electrical security, the back up filament is likely to be close to the coolest portions of the lamp envelope. The result is the first filament evaporates during normal operation, and the envelope around the second filament is darkened by the depositing material. The second filament is then obscured by the deposition of the materials from the first filament. There is then a need to protect the light path for the back up filament from being degraded by deposited materials during the operation of the first filament.
In a similar fashion, as the first filament ages, the envelope fill gas may be degraded, either by emitted material, filament failure, chemical saturation, or leaked gases. The second filament is therefore likely to operate in a less than ideal fill gas. There is then a need to protect the fill gas for the second filament from being degraded by operation of the first filament.
Numerous U.S. patents show examples of double filamented lamps. U.S. Pat. No. 1,581,690 to A. L. Powell for a Two-Filament Street Series Lamp shows two filaments in a single envelope with means for connecting the second filament when the first filament fails. U.S. Pat. No. 1,713,753 to F. Eckhardt et al for an Electric Incandescent Lamp shows a lamp with two filaments in a single envelope allowing a selection of filaments according to the applied voltage. Other U.S. patents showing double filamented lamps includes at least the following 1,717,283; 1,859,661; 2,029,211; 2,074,246; 2,084,176; 2,161,443; 2,862,147; 3,327,162; 3,319,115; 3,458,756; and 3,697,802.